This invention relates to a brake servo booster, primarily for use in providing power assistance in the actuation of a brake master cylinder of a vehicle hydraulic braking system capable of operating in a traction control mode, the booster being of the general kind in which the supply of air to a servo piston, usually in the form of a diaphragm assembly, is controlled by a valve mechanism which includes a reciprocable valve element operatively connected to a force input member actuated by a driver-operated control.
For the traction control mode, the booster is actuated automatically to apply the brakes without intervention by the driver and the connection between the force input member and the booster valve element must be such as to allow this to happen without significantly affecting the position of the brake pedal. An object of the invention is to provide a simple and convenient form of connection for this purpose.
According to the invention, the force input member of a booster includes two parts of which a first one is connected, in use, to the driver-operated control, and the second one to the valve element, the arrangement being such that operating force applied to the control member is transmitted through both of said first and second parts of the input member to the valve element, whilst operation of the booster in the traction control mode is permitted by the second part of the force input member moving relative to the first part.
In a preferred arrangement, the first and second parts of the force input member are telescoped together with one of the parts slidingly housed within a bore of the other part.
Typically, the valve element is housed within a hub connected to the servo piston, and the hub carries abutment means which engages a housing part of the booster to set the retracted position of the first part of the input member and thereby of the control member.